Implement FindTaskCommand for TodoList

[jonasngs]

Fixes #175

This PR implements a command to find tasks using a set of search parameters.

The following classes were added:

1. FindTaskParser: parse search parameters
2. FindTaskCommand: execute command and update filteredTaskList according to the predicate
3. FindTaskCriteria: encapsulates a list of predicate to test a contact with
4. NameContainsKeywordsPredicate: predicate to test a task's name
5. TaskContainsTagsPredicate: predicate to test a task's tags
6. TaskMatchesDatePredicate: predicate to test a task's date
7. TaskMatchesPriorityPredicate: predicate to test a task's predicate

[davidcaiqifan]

LGTM. The code looks clean and well written with just a few inconsistencies.

[codecov-io]

Codecov Report

Merging #419 into master will decrease coverage by 0.47%.
The diff coverage is 12.50%.

@@             Coverage Diff              @@
##             master     #419      +/-   ##
============================================
- Coverage     18.72%   18.24%   -0.48%     
  Complexity      329      329              
============================================
  Files           205      210       +5     
  Lines          3776     3874      +98     
  Branches        458      477      +19     
============================================
  Hits            707      707              
- Misses         2991     3090      +99     
+ Partials         78       77       -1     

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/seedu/address/MainApp.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...ommands/contactlistcommands/AddContactCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...ands/contactlistcommands/DeleteContactCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...mmands/contactlistcommands/EditContactCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...nds/contactlistcommands/EditContactDescriptor.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...mmands/contactlistcommands/FindContactCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...mmands/contactlistcommands/ListContactCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...gic/commands/todolistcommands/FindTaskCommand.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...a/seedu/address/logic/parser/ModuleListParser.java	48.27% <ø> (ø)	10.00 <0.00> (ø)
... and 21 more

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